1. Field of the Invention
The present invention relates to a process and apparatus for the thermal treatment of powdered minerals which are suspended in a gas. More specifically, it relates to processes and an apparatus where the materials ar preheated and are treated in suspension in a current of warm gases produced by the combustion of air and a fuel injected into a furnace. There are two groups of cyclone separators, one group placed upstream of, and one group placed downstream of, the furnace. The cyclones are connected together and to the furnace by means of pipes arranged so that the exhaust gases from the furnace pass successively into all the cyclones of a set. A current of air traverses all the cyclones of the other set before entering the furnace in which it is used as combustion air.
Heat transfer between the materials to be treated and the furnace exhaust gases, on the one hand, and between the treated materials and the air, on the other, are accomplished partially in the cyclone and partially in the pipes where the materials are suspended in a gas. The cyclones separate the material from the gaseous current. The material is again placed in suspension in an upstream portion of the gas circuit, so as to achieve counter-current heat transfer throughout.
2. The Prior Art
At the outlet of the first set of cyclones, where the materials are preheated, the exhaust gases pass into a filter, generally an electro-filter, before being released into the atmosphere. Most of the dust collected in this filter is reintroduced into the apparatus. In order to avoid clogging of the filters by progressive accumulation of dust from the gas circuit, a portion of the dust is either released to the atmosphere, or mixed with the final product.
This release of dust results in a production loss. The mixing of the dust from the filters with the final product causes the latter to contain a small, but not negligible, percentage of dust which has not been treated. For certain applications, this may present a disadvantage. For example, in the calcination of alumina, the presence of alumina hydrate in the finished product may be troublesome. Such a result may also be unacceptable in the calcination of phosphates.